The Beauty of Fluid Mechanics In Video and Photos

An anonymous reader writes "F/A-18 Hornet jet fighters just having some fun — and making science interesting at the same time: Video #1, #2, #3, and a photo gallery. The formal name for the cool 'vapor cone' is Prandtl-Glauert Condensation Cloud, which is due the Prandtl-Glauert singularity."

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Re:How does it follow the jet?

[pclminion]

How/why does the condensation follow the jet, instead of leaving a trail like you would expect? (Or at least like I would expect).

If you read the article, it is explained that the condensation occurs because the air cools in regions of transient extremely low pressure. These regions are "attached" to the aircraft and so the cloud follows it. When the aircraft passes, the air returns to its equilibrium pressure, the temperature increases accordingly and the cloud evaporates.

Also, what the hell is wrong with Slashdot these days when the only articles that get over 500 comments are political or Sony-related and something cool like this gets less than 10 comments?

Re:How does it follow the jet?

[Profane MuthaFucka]

The temperature is directly related to pressure. The pressure changes very quickly, so the temperature changes just as quickly. The condensation of water into a cloud is directly related to the temperature and the pressure. If you have the right conditions, it's going to be a cloud. The time it takes for the water vapor to condense into a cloud is related to the speed of the molecules of water vapor. They're moving pretty fast. So quickly that the 700MPH jet is practically standing still by comparison.

So, ultimately the answer to why the water vapor condenses and evaporates so quickly is because that's how fast that process happens. You just never think about it as a fast process because every day process of producing water vapor clouds, such as boiling water, are limited by the slow pace of thermodynamic conduction of heat into the water.

Re:How does it follow the jet?

[Tmack]

How can the air so quickly return to the state that it was in so as to immediately absorb (I don't know the proper term) the condensation?

Thermodynamics... Pressure, Temperature and Volume are all related. Sound is nothing more than pressure waves, and the aircraft moving through the air at close to Mach speeds will create very interesting pressure waves around it, that can extend quite far from the craft itself. Since the source of the pressure wave is the aircraft itself, and moving at close to the speed of sound (speed that pressure waves propagate), the pressure wave will become a standing wave with respect to the aircraft. As substances compress (increase pressure in the same volume), thermodynamics regulates that temperature will rise, and as they expand (pressure drops), temperature will drop. These same principals are what cause your Air conditioner and refrigerator to work. The atmosphere also has what is known as the dew point, which simply stated, is the temperature that a cloud of condensation will form. When the ground temperature is at the dew point, you get fog. It is a temperature based on pressure and humidity. Since the aircraft causes radical changes in temperature and pressure, if you get the right combination of conditions, the aircraft's movement through the air can cause a local zone of air that will hit the dew point and create the cloud. It is when it hits this point that the cloud will magically appear. The thermal "momentum" for the change is dictated by the specific heat of the air, and the heat of vaporization for the water in the air at the location (amount of energy needed to change its temp, and energy required to change phases of the water). Since the energy is being provided by the aircraft, there is no shortage, and it is powerful enough to drive these changes very quickly, which is why it appears and disappears so fast.

Besides my contorted description here, the referenced articles do a decent job going into more detail. There are also some nice java applets that can visualized the pressure waves created at near-mach speeds to demonstrate why the shape of the cone is as large as it is (mainly due to the amplification predicted by the Prandtl-Glauert singularity equations). I tried looking for them but couldnt find the one I remember seeing before..

tm

Re:How does it follow the jet?

[mikael]

Because the air is being rapidly compressed, then decompressing, before returning to normal pressure.

Any object moving through air creates sound waves. For an aircraft travelling close to the speed of sound, these sound waves build up into a pressure wave of compressed air in front of the aircraft. If you compress air, the pressure goes up and the air becomes warmer (the faster you go, the more compressed the air becomes). But every compression wave also has a decompression wave behind it. When air pressure is reduced, the temperature goes down. In this case, this is enough to bring the air temperature below the condesation point of water, and so the moisture in the air briefly condenses into a cloud. Once pressure is restored (regaining a bit of heat), the water droplets immediately dissipate back into vapour.

These photographs and videos were taken in a hot humid environment that was on the coast or at sea.

Re:How does it follow the jet?

[Anonymous Coward]

Have a careful look at this [galleryoff...hanics.com] famous and popular video of a transonic F-14 Tomcat (best viewed frame by frame): http://www.galleryoffluidmechanics.com/conden/f14. mpeg [galleryoff...hanics.com]

Why does the cloud begin to form above the jet? Are there any insights you can bring that can help solve this mystery? In his explanatory remarks at http://www.galleryoffluidmechanics.com/conden/mpeg f14.htm [galleryoff...hanics.com] Dr. Mark S. Cramer (who wrote the Prandtl-Glauert Condensation Cloud material pointed to in this excellent and very interesting /. article) says that

"A second feature is the fact that the condensation first appears above the aircraft when the cloud reforms (and possibly when it forms). Of course, it is expected that the cloud should first form in the low pressure region which is necessarily above any lifting aircraft. However, the largest pressure perturbations are expected to be very near the aircraft. Hence the lowest pressures and therefore the cloud formation ought to be at the aircraft surface. We can give the usual atmospheric turbulence arguments, but without much conviction. Bob Harrington did point out that there may be flow disturbances generated by the ship which could cause an unanticipated Mach number or pressure distribution. I like this better than the generic turbulence argument."

In simple language, Who Knows!